1. Technical Field
The invention relates to liquid electrophotographic printing. In particular, the invention relates to a composite coating for substrates used in liquid electrophotographic printing.
2. Description of Related Art
Digital printing provides numerous options not previously available to consumers. Digital printing can create a printed image directly from digital data. For example, a desktop publishing program can provide text and images in an electronic layout that is transferred to a substrate in a printed format. In the digital printing process, every image can be varied electronically to allow for quick and dynamic generation of printed information. Digital offset printing is a type of digital printing that uses an offset cylinder. With respect to some digital offset presses, electronic documents are transferred digitally from workstations directly to the press. Some steps associated with conventional offset printing and their associated costs, such as film output, film assembly and plate processing are eliminated. The offset cylinder associated with some digital printing presses may protect a printing plate of the digital printing press. Moreover, such offset cylinders may extend the life of the printing plate. Further, the offset cylinder may compensate for unevenness in a printing surface of the substrate to be printed. As such, digital offset printing can be used for a wide variety of substrates.
Digital electrophotographic printing is a form of digital printing that is also known as electro-digital printing (EDP). A form of EDP is known as liquid electrophotographic (LEP) printing. Liquid electrophotographic (LEP) printing is different from conventional inkjet printing and laser digital printing in that LEP printing uses a liquid toner based ink, herein referred to as an ‘LEP ink’ as opposed to a dry toner based ink.
Substrates used in LEP printing include, but are not limited to, paper, various plastics and metal. The substrates may be coated or uncoated. Various substrate coatings may be used, for example, to improve the substrate appearance, to improve image quality of a printed image, and to improve substrate durability during digital printing. For example, a paper substrate may have a coating that is applied by paper manufacturers to strengthen the paper substrate for printing. At the paper manufacturing level, much has been done to improve adhesion between a substrate and such coatings.
However, less has been done to improve adhesion between a coated or uncoated substrate and the LEP ink used in LEP printing. Instead, some manufacturers offer a treatment or primer that either the user applies to a substrate to be printed before or during LEP printing or a substrate manufacturer applies to the substrate. When applied by the user, it is an added step in the LEP printing process. When applied by the substrate manufacturer, the treated or primed substrates may have a limited shelf-life. The surface treatment or primer is designed to improve adhesion between the LEP ink and the substrate. While very effective at the LEP printing level, improved LEP ink adhesion to LEP-compatible substrates should be addressed at the substrate manufacturing level instead of by the user at the digital printing level.